Abstract
Many gold placers located in the Coastal Range of south-central Chile have been exploited commercially during the past decades; nevertheless and until now, there are not any exploration studies of these placers and their basement. In order to provide an overview of exploratory potential, the stream sediments that are part of several gold placers in the northwest area of the BioBío region are analyzed, which have been registered on the basis of empirical evidence from artisanal mining activities. The overview consists of two goals: 1) determine the presence of auriferous occurrences (stream sediments and basement) and 2) identify potentially favorable areas to find gold placer deposits. Through optical microscopy this study characterized the metallic and non-metallic mineralogy of the basement, along with geochemical analyses of stream sediments and surrounding rocks of the placer deposits. The geochemical results show two sectors with high precious metal contents in sediments (up to 24 ppm of Au and 8 ppm of Ag) and two rocky outcrops with Au (757 and 41 ppb). Additionally, there are depressions in the study area that have received large amounts of sediments (reaching up to 170 m-thick), which are favorable places for the accumulation of auriferous sediments from nearby valleys due to the tectonic and structural conditions where they are located. Then, these results show an exploratory potential for gold placers and can be useful to delineate future exploration strategies in the Coastal Range of south-central Chile.
Highlights
IntroductionThe National Gold Plan (Plan Aurífero Nacional de Chile) conducted by the Chilean National Mining Corporation (ENAMI) between years 1983-1987 allowed recognizing the gold placers along the country [1], its depositional setting [2] and grouped them into four categories: 1) placers related to Upper Jurassic-Lower Tertiary granitoids, 2) placers related to Paleozoic Metamorphic Basement and/or South Coastal Batholith, 3) placers related to quaternary moraines, and 4) placers located at oceanic beaches
Many gold placers located in the Coastal Range of south-central Chile have been exploited commercially during the past decades; and until now, there are not any exploration studies of these placers and their basement
In order to provide an overview of exploratory potential, the stream sediments that are part of several gold placers in the northwest area of the BioBío region are analyzed, which have been registered on the basis of empirical evidence from artisanal mining activities
Summary
The National Gold Plan (Plan Aurífero Nacional de Chile) conducted by the Chilean National Mining Corporation (ENAMI) between years 1983-1987 allowed recognizing the gold placers along the country [1], its depositional setting [2] and grouped them into four categories: 1) placers related to Upper Jurassic-Lower Tertiary granitoids, 2) placers related to Paleozoic Metamorphic Basement and/or South Coastal Batholith, 3) placers related to quaternary moraines, and 4) placers located at oceanic beaches. Evidence of gold occurrences in rocks of the Paleozoic basement in south-central Chile is very scarce; some of them are located near gold placers (e.g., [3]). Banded Iron Formation (BIF) type deposits hosted in rocks of the Metamorphic Basement [8] [9] have been detected close to the Mahuilque gold placer (Figure 1); these show Au contents (up to 0.36 ppm) mainly in metacherts [3] and are similar to Au-rich BIF deposits of south-central Africa [10]
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